1. Field of the Invention
This invention relates to an electronic sewing machine, and more particularly relates to a sewing machine having an electronic memory which stores pattern data for producing selected stitch patterns by way of pulse motor controlled stitch forming devices. Such a sewing machine employs two clutches, the first arranged between one of the pulse motors and a needle hole changing mechanism, and the second arranged between another pulse motor and a feed dog dropping mechanism, so that the pulse motors may change to needle hole and feed to cloth in accordance with the stitch desired.
2. Description of the Prior Art
When the upper shaft of a sewing machine is rotated at 1000 rpm to stitch a pattern, the time required for one rotation is 60 mS (milliseconds). Therefore, it is necessary to drive the pulse motors which drive the needle and feed dog each 20 mS, which is less than the time required for the upper shaft to rotate 180.degree.. Since it may take as many as 78 steps for the pulse motor to go between extreme positions, the pulse motors must be rotated at a high speed. In this case, the output torque of a pulse motor need only exceed the sum of (a) the torque required to overcome inertia of both the driven load and the pulse motor itself and (b) the torque required to overcome friction resistance. This output torque may be relatively small. On the other hand, for operation of the needle hole changing mechanism or the feed dog dropping mechanism, the output torque of the pulse motor must exceed both the torque requirements listed above and a further resistance torque caused by the frictional load and springs. This latter case requires considerably greater output torque, in comparison with the torque required only to stitch a pattern. Therefore, if the pulse motor is rotated at a high speed, as it does when producing a stitch, output torque of the pulse motor is reduced. If the output torque of the pulse motor is to be increased at high speed, it becomes necessary to provide a larger pulse motor. This is, however, very inconvenient in view of the limited space within a sewing machine.
The present invention has been provided to solve this problem.